diag: Donot open mhi channels on usb connect

Donot open mhi channels during usb connect since
mhi channels are not closed during disconnect.
Mhi channels are opened/closed based on mhi
probe/remove calls.

Change-Id: Ie297a3fb8cf1e6c8d28af02e5d7bdf7ef4ee0fee
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>